Such an apparatus and such a method are known from EP 0 829 722 A2. Said known apparatus is an atomic force microscope (AFM). Said known atomic force microscope comprises a measuring tip acting as measuring head, which is coated, either directly or after application of a metal- or oxide layer, with one or more probe molecules. The measuring tip is brought into contact with the surface of the sample to be examined, and the force occurring during retracting is measured. Thereby qualitative differences in the interaction between probe molecules and surfaces can be measured.
A corresponding method for a chemically differentiating imaging by use of atomic force microscopy is known from EP 0 727 639 A1.
Such atomic force microscopes can be used for imaging the topography of surfaces as well as for measuring binding forces between antibodies and antigens, as well as for a chemically differentiating imaging. More detailed information is to be found in the publication of FLORIN, E. L., MOY, V. T., GAUB, H. E.: Adhesion forces between individual ligand-receptor pairs, Science, 264, 415 (1994), in the publication DAMMER, U., HEGNER, M. et al.: Specific antigen/antibody interactions measured by force microscopy, Biophys. J. 70, 2437 (1996) as well as in the publication SMITH, D. A., CONNELL, S. D. et al.: Chemical force microscopy: applications in surface characterisation of natural hydroxyapatite, Analytica Chimica Acta 479, 39 (2003).
Force plateaus in the force-distance curve were discovered and interpreted a few years ago. Initial experiments on this issue are described in the publication CHÂTELLIER, T., SENDEN, J. et al.: Detachment of a single polyelectrolyte chain adsorbed on a charged surface, Europhys. Lett., 41, 303 (1998), in the publication HUGEL, T., GROSHOLZ et al.: Elasticity of Single Polyelectrolyte Chains and their desorption from solid supports studies by AFM base Single Molecule Force Spectroscopy Macromolecules, 34, 1039 (2001), as well as in the publication HUGEL, T., SEITZ, M.: The Study of Molecular Interactions by AFM Force Spectroscopy Macromol. Rapid Commun. 22, 989 (2001). In the scope of these experiments, polymers were adsorbed to a surface and then pulled off with a functionalized measuring tip. Here, two problems occur. On the one hand, the tip is after a short time not usable anymore, because the reactive groups are saturated. On the other hand, several hundreds of force-distance curves have to be taken until an outright single molecule curve is measured, since not in every measurement a polymer is received by the measuring tip.
For solving these problems, several probe molecules were covalently bound to the measuring tip and then brought into contact with the surface. After a certain time the tip was pulled off from the surface and the balance force necessary for desorption was measured. This cycle could often be repeated several dozens of times. Experiments of this kind are described in the publication SEITZ, M., FRIEDSAM, C., et al.: Probing Surfaces with Single Polymers ChemPhys-Chem. 4, 986 (2003) and FRIEDSAM, C., DEL CAMPO BECARES, A. et al.: Polymer Functionalized AFM tips for Long-Term Measurements in Single-Molecule Force Spectroscopy, ChemPhysChem. 5, 388 (2004).
A further great problem with conventional methods consists in discerning the intramolecular forces to be measured from unspecific interactions, in particular from interactions between the measuring tip and the surface to be measured. This problem can be reduced by flexible linkers, but at the best in exceptional cases be resolved. Experiments of this kind are described in HINTERDORFER, P., BAUMGARTNER, W. et al.: Detection and localization of individual antibody-antigen recognition events by atomic force microscopy, PNAS 93, 3477 (1996) and in ROS, R., SCHWESINGER, F. et al.: Antigen binding forces of individually addressed single-chain Fv antibody molecules, PNAS, 95, 7402 (1998).
A theoretical explanation for the occurrence of the observed adhesion forces on the basis of statistical mechanics is for certain systems given in HANKE, F., LIVADARU, L., KREUZER, H. J.: Adsorption forces on a single polymer molecule in contact with a solid surface, Europhys. Lett., 69 (2), 242 (2005).
DE 102 08 800 A1 further describes a method for measuring adhesion properties with an atomic force microscope. It describes to use a frequency shift in the dynamic atomic force microscopy for measuring the adhesion. This is an indirect measuring, since the frequency-distance curves needs to be converted in a force-distance curve beforehand. Moreover, this method could only be demonstrated in ultra high vacuum.
In the field of medical technology, the inspection of the coating of implants is of particular importance, and on the chemical field so is the inspection of polymer coatings. In both application fields, until today atomic force microscopes have not been used.
Today there are existing several mechanical testing methods for examining the adhesion of different coatings though. But the application field is very restricted, in particular for polymer coatings, and involves some disadvantages. Almost all these testing methods are not non-destructive—for example a scratch test, a indentation test, a tensile test or a four-point-bending test—or can only be used restricted to organic coatings—for example an inflation method or a peeling method. In some cases, for example for a tensile test or a peeling test, the use of an adhesive is necessary, wherein it cannot be excluded that the solvent contained in the adhesive reacts with the organic coating. Due to the generation of stress zones, which is associated with the contact of the probe to the coating surface to be examined, the tests on the mechanical adhesion can hardly be carried out under the same conditions and with a satisfactory repetition rate. Due to plastic deformations of substrate and coating, which are generated by the measuring head of the majority of the experimental systems, the value experimentally found for the adhesion strength often differs from the actual value by several orders of magnitude. Quantitative statements concerning the adhesion can in all cases not be made without a high uncertainty.